Communities

IET Communities provide like-minded people with opportunities to share ideas, collaborate, learn and network. With more than 100 Local and Technical Networks around the world, you can feel confident of finding a community that suits your interests.

Topic Title: LED bulb efficiency - its all about the drivers not the LEDs? Topic Summary: ET&T : Are LEDs still the future for lighting?Created On: 24 August 2012 09:10 AMStatus:
Read Only Related E&T article: Are LEDs still the future for lighting?

Last year we dissassembled a GU10 LED bulb - I forget which manufacturer it was. I was curious to find out what kind of driver circuit was being used to "control" the current through the LEDs, which is quite a difficult thing to do (cheaply) given a 220-250V AC supply. I thought I might find a cheap switched-mode circuit or something like that.

Instead, what we found was that about 80% of the electrical energy was dumped into a simple resistor which was used as a crude "constant current" device. In this case, whether the LED is 60 or 99% efficient becomes almost irrelevant - its the efficiency and cost of the driver circuit which is important, and making more efficient, reliable drivers at such small power levels from 220-240VAC, within the cost constraints, is pretty hard.

Maybe a 24V DC low-power domestic ring, specifically for LED lighting (and nothing else) is the way to go ... it would make the driver circuits much easier to build?

Nothing new in that Andrew - look back far enough or deeply enough and you'll find plenty of dimming devices using power loss resistors - remember the tungsten or resistor ballasted fluorescent lamp circuits - you just simply dropped what voltage you didn't need for the fluorescent across a resistance - if it was a tungsten lamp you got some light (and heat) - if it was a resistor, you just got heat - sometimes the heat was useful, in many cases it wasn't - if only due to diurnal cycles

Is the economics of a 24V lighting ring in domestic houses worthwhile - I doubt it (at least for many years to come)

I've seen some cheap-and-cheerful AC step-down circuits, where electrical isolation was not required, using the reactance of a small film capacitor to drop the voltage. Doesn't help the power-factor but probably more energy efficient, overall, than using a simple resistor.

The DC power converter could be specified by the number of separate lighting circuits it can power and have current limiting and earth leakage detection fusing, and power saving mode when no lighting is required.

Are there any reasons (safety, fire risk etc) why this sort of conversion to an existing lighting circuit should not be allowed from a wiring regulation point of view?

You could piggy back the lighting control comms, signals on top of the DC supply, to give dimming and colour change controls for higher spec installations.

Originally posted by: aroscoeInstead, what we found was that about 80% of the electrical energy was dumped into a simple resistor which was used as a crude "constant current" device. In this case, whether the LED is 60 or 99% efficient becomes almost irrelevant - its the efficiency and cost of the driver circuit which is important, and making more efficient, reliable drivers at such small power levels from 220-240VAC, within the cost constraints, is pretty hard.

LED lighting is something my son has been investigating for some time. His results from power consumption tests carried out on commercially available LED bulbs are similar to your own findings - in that it is common for more power to be dissipated in the driver circuit rather than the LED itself. He hasn't quite reached a conclusion on whether reducing power consumption of LED drivers will have a minimal impact on overall energy consumption from lighting compared to exchanging all less energy efficient bulbs for LED lighting, or whether it is something that manufacturers must take more seriously.

LEDs can be driven either from a constant current source, PWM, or both. The method used affects the colour of the LED during dimming. There is quite a good article in a magazine about LED drivers and if anybody is interested I will try and find it.

Maybe a 24V DC low-power domestic ring, specifically for LED lighting (and nothing else) is the way to go ... it would make the driver circuits much easier to build?

Originally posted by: jarathoonWhat about keeping the 240V ring main and unhooking the the upstairs and downstairs lighting circuits etc at the consumer unit and efficiently transforming and regulating to 24V DC there?

Low voltage domestic lighting using incandescent bulbs is commercially available but it has never been popular in Britain. Most systems work on 12V AC rather than 24V DC supplied from a high quality transformer. A low voltage LED lighting system with a driver circuit in each bulb would not require such a high quality transformer. Altervatively a switched mode PSU could be used. It would be quite easy to incorporate a battery back up into a low voltage lighting system to enable it to function during power cuts.

Are there any reasons (safety, fire risk etc) why this sort of conversion to an existing lighting circuit should not be allowed from a wiring regulation point of view?

Are there any IET wiring regulations for low voltage domestic supplies? If existing wiring is used then the maximum current will have to be limited to 5A as that is the norm for domestic lighting circuit breakers . Assuming a 12V supply then 60W is the maximum power per lighting circuit.

You could piggy back the lighting control comms, signals on top of the DC supply, to give dimming and colour change controls for higher spec installations.

The possibilities are endless. Over the years my son has designed and produced prototypes of a number of controllable lighting systems. Each LED driver circuit is allocated a code and data can be sent along power wires for switching and dimming. Security features could easily be incorporated such as particular lights that switch on between sunset and 11 PM or lights that switch on and off randomly to give the impression that the house is occupied.

From what I have found it appears perfectly safe to set a 60V DC standard for lighting circuits, (especially so if earth leakage detection trip are included as standard). That would allow up to 300W on a single 5A lighting circuit, more than enough for most houses, if the LED driver effficiency is as low as you suggest.

They are claiming Typical DC/DC conversion efficiency up to 98 percent and they give pricing levels.

Using DC/DC conversion techniques to DIM LED's might be better anyway, as I've heard that some people can see the modulated light given out by some of PWM dimmable LED lights currently on the market.

A switch mode power supply at the consumer unit giving out 60V can be over 80% efficient, and together with a DC/DC converter local to the LED lamp we get a power transmission efficiency in the 60-90% range, rather than the 20-30% range that you say is the norm at the moment.

"Why is the standard set for 24 Volt DC?
Power over 30 Volts is not considered "safe" when conductors are exposed (without insulation) and must be enclosed in metal jackets, conduits and enclosures. Power less than 24 Volts (i.e. 12 Volt DC systems) can create more significant power losses in the wiring. The selection of 24 Volt DC provides the dual benefits of flexible, modular wiring and safe, efficient power distribution. Also, many digital devices used within commercial interiors today that inherently use DC power - such as occupancy and day lighting sensors - are already based on this voltage."

This appears to be guff and nonsense to me as no power system can be run without insulating and protecting conductors. In regards to safety the PoE standard allows for a voltage of 57V DC and BT runs 50V DC on its telephone lines. Has anyone ever be injured coming into contact with these sorts of voltages?

I haven't yet found a reason why DC power for lighting circuit upgrades should not be in the 50 to 60V DC range giving up to 300W of lighting on a standard 5A lighting circuit.

Such a conversion standard will be suitable for use in smaller industrial premises as well.

What are the possibilities for converting 50-60V DC to 30KHz high frequency (HF) AC for fluorescents?

Energy savings seem possible by using a 30KHz HF supply rather than a 50Hz supply. (with no flicker) Some DC to DC converters use intermediate 30 KHz switching frequency anyway, so are small low cost efficient 60V DC to 30KHz HF supplies feasible for fluorescents?

How do the ballast requirements and costs change when powering from 60 V DC?

I think DC distribution in houses is the way forward, its safer, gets rid of wall warts and is more efficient, most electronic equipment has a PSU consisting of a transformer, rectifier and smoothing circuit, This would be made much simpler if the supply was 60v DC.
Also renewable energy is normally generated at DC and converted to AC instead it could feed directly onto the DC system.

The main delay as far as I can see is agreeing on the voltage and plug type. Obviously 230v would still be required for high power items.

Energy savings could be made at high frequencys but volt drop becomes a problem, inductive impedance is 2PI F L so increasing the frequency increases the impedance. 400Hz is used on aircraft as the machines are lighter and the cable runs are short.

I think DC distribution in houses is the way forward, its safer, gets rid of wall warts and is more efficient, most electronic equipment has a PSU consisting of a transformer, rectifier and smoothing circuit, This would be made much simpler if the supply was 60v DC.

Also renewable energy is normally generated at DC and converted to AC instead it could feed directly onto the DC system.

The main delay as far as I can see is agreeing on the voltage and plug type. Obviously 230v would still be required for high power items.

Energy savings could be made at high frequencys but volt drop becomes a problem, inductive impedance is 2PI F L so increasing the frequency increases the impedance. 400Hz is used on aircraft as the machines are lighter and the cable runs are short.

My point is that setting new DC standards (and any piggy backed comms signals) for lighting circuits should be started sooner rather than later because of the huge energy savings that can potentially result.

I think initially aiming to make the DC system standards backwards compatible with current AC lighting (wiring and current carrying) standards, means that conversions can start being made quickly and cost effectively (i.e. without ripping out all the existing wiring first).

In regard to light dimming and colour balance control: I have in mind that the necessary control circuitry (a DC-DC converter) is in the light fitting itself. All the dimmer switch or central controller does is send standardised digital commands using the piggy backed comms interface.

In regard to light switching: this can be done either as normal with a conventional switch or if the switch is left on via an optional digital comms signal to the light fitting.

In regard to high frequency outputs for fluorescents the DC-HF AC can be done locally in the lamp unit using DC to high frequency AC converter + starter module as swap in replacements for the existing array of fluorescent power supply modules.

This way if companies and home owners want to upgrade existing fittings all they have to do is change power supply/ballast modules and lamps, rather than having to rip out all the light fittings themselves.

Whether energy efficient DC lighting standards for converting existing systems should be delayed until we find agreement on how new systems is something to be debated. How might setting these backward compatible system conversion standards first constrain or unnecessarily distort future standards for new lighting system standards? Is more than one DC standard necessary?

I would go for 60V DC, because it is safe and has long standing historical pedigree (from the Babylonians because it is one of the most practical numbers for people to work with because 60 as you will know has prime factors 2^2, 3, 5 )

I believe that at these voltage levels using conventional light switches may be a problem due to arcing. This was found to be a problem during the development of the 42V car electrical systems.

Similar problems may apply to circuit breakers and fuses.

Best regards

Roger

The ac circuit breakers and fuses would feed into the Switched mode DC power supply module, so that could be make compatible with existing circuit breakers including RCD's

So how do you fix the light switch problem cost effectively?

I think you are right you would start rating light switches according to their load switching/digital interoperability features.

- For really low cost systems (with very frugal lightling needs) you would have to specify in the wiring standards the maximum size and type of DC load (resistive/capacitive/inductive) an existing 240V AC switch could switch.

- Solve the problem digitally, by doing all switching via the piggy backed digital signalling, so that cheap light switches can be produced that don't do any power switching at all.

Solving the problem digitally allows fire alarm systems to switch on secondary non-safety critical lighting automatically in the event of a fire.

Safety critical lighting could be set up to receive heart beat signals every second or so, to keep them off. If these heartbeat signals were lost for some reason the emergency lighting could be set to automatically come on after a predefined timeout, say 5 or 10 seconds.

24V (at 5A) gives 120W per existing lighting circuit, which won't be enough to achieve backwards compatibility in medium to large houses using the existing 5A wiring. (For example to power large chandeliers in houses or hotels, that have been modified to use LED's )

It is obviously a lot easier and cheaper to change switches than rip-out existing wiring.

There are normally under 10 switches per floor in a house, so for all but the largest houses these can be swapped out as part of one person day's on-site work.

Prohibiting the use of existing mechanical AC light switches with 60V DC lighting makes things very simple and minimises the scope for confusion and mistakes.

I think once you have set the voltage standard (60V DC) and components for this system start being mass produced, there will be little need for lower voltage consumer standards that ultimately need larger gauge copper wire to transmit the same power (other than the existing 12V (or multiples there of) systems for cars and caravans).

Specialist industrial standards need to be set differently because of the need for higher powers or intrinsically safe standards (ignition free) for electical equipment used in hazardous areas.